返回值可接受为空，用以表示状态失败

举个栗子，

#include <iostream>
#include <optional>

using namespace std;

class User{
    string name;
    optional<string> nickName;
    optional<int> age;
    public:
    User(const string& name,optional<string> nickName,optional<int> age):name(name),nickName(nickName),age(age){}
    friend ostream& operator << (ostream& stream,const User& user);
};

ostream& operator << (ostream& os,const User& user) {
    os<< user.name << ' ';
    if(user.nickName) {
        os<< *user.nickName << ' ';
    }
    if(user.age) {
    os <<"age of" << *user.age;
    }
    return os;
}

int main() {
    User peter("Peter", "Spiderman", 20);
    User chris("Chris", nullopt, nullopt);

    cout << peter << endl;
    cout << chris << endl;
    return 0;
} 

Peter Spiderman age of20
Chris